In recent years, with increasing concerns on air pollution, various efforts has been being made for reducing nitrogen oxides (NOx) from internal combustion engines in industrial boilers, vehicles or the like. The most widely used commercial technique for removing nitrogen oxides is currently SCR (Selective Catalyst Reduction) technique in which nitrogen oxides are converted, through a catalytic reduction reaction, to harmless substances, i.e. water and diatomic nitrogen by using ammonia as a reducing agent. However, the reducing agent, ammonia has disadvantages such that its transport, storage and handling are difficult, owing to its explosiveness, unpleasant strong odor and toxicity. Accordingly, techniques using solid urea or an aqueous urea solution which is relatively easy to transport, store and handle have been attracting attention. Urea has a chemical structure in which two amino groups are joined by the carbon of a carbonyl (—C═O) group, and is converted to ammonia which makes possible to selectively convert nitrogen oxides to N2, in the event of exhaust gas emission from an internal combustion engine of a vehicle equipped with a SCR system.
However, some of ammonia converted from urea fails to participate in the reductive conversion of nitrogen oxides to N2 and is released into the air, thereby raising an additional problem of being another cause of air pollution.
Concerning its inflammability and toxicity, ammonia-containing exhaust gas should not be released into the air as it is, and needs further treatment for removing ammonia so as to eliminate its dangerous and toxic properties.
There are wet processes, combustion processes and dry processes in exhaust gas treatment methods. In the wet processes, exhaust gas is cleansed by chemical liquid; in the combustion processes, exhaust gas is further burned at a high temperature by a burner or the like, thereby resulting in harmless gas; and in the dry processes, a stream of exhaust gas passes through a column charged with a solid treatment agent or a decomposition catalyst, wherein the gas to be treated is absorbed to the treatment agent via chemical interactions therebetween, i.e. adsorption and/or chemical reactions, or is converted to harmless substances by the catalyst. The dry processes are often used in the treatment of exhaust gases such as metal hydride-containing gas, halide-containing gas or ammonia-containing gas.
In the meantime, when using the wet processes, waste water from the process contains ammonia, and thus it requires further waste water treatment. The combustion processes also have a problem such that NOx generated from ammonia combustion needs to be further treated.
Many techniques related to an ammonia-reducing catalyst for reducing ammonia through the dry processes have been known in this field of art. For example, as ammonia-reducing catalysts, Japanese Laid-Open Patent Publication No. Hei 11-042422 discloses copper oxide, chrome oxide, manganese oxide, iron oxide, palladium, platinum and the like; Japanese Laid-Open Patent Publication No. Hei 07-328440 discloses chrome, copper or cobalt supported on zeolite; or Japanese Laid-Open Patent Publication No. Hei 10-249165 discloses metal elements of Group 8 and/or Group 1B in the periodic table of elements (sub-Group format).
However, although the catalysts prepared from such copper oxide, chrome oxide, manganese oxide, iron oxide, palladium or platinum have an excellent ammonia reducing rate, these catalysts do not yield N2 with high selectivity, but also generate nitrogen oxides such as N2O, NO2 and NO, thereby causing further environmental problem.
Moreover, regarding the catalysts in which chrome, copper or cobalt is supported on zeolite, although the object thereof is to reduce ammonia, while inhibiting the generation of nitrogen oxides as much as it possible, the gas to be treated was found to have only very low concentration of ammonia such as 30 ppm and also a very low concentration of oxygen such as 2%. It means that the catalysts are only effective in very restricted conditions. Since the ammonia concentration in exhaust gas from a semiconductor manufacturing process or the like are very high as several %, and the oxygen concentration is also as high as the oxygen concentration in the air, generation of nitrogen oxides such as N2O, NO2 and NO cannot be avoided when using the above supported catalysts.
The catalysts containing metal elements of Group 8 and/or Group 1B in the periodic table of elements (sub-Group format) may reduce ammonia at a low temperature without generating nitrogen oxides due to the presence of hydrogen as a reducing agent, however it is disadvantageous in terms of process economy since it requires hydrogen as an essential component.